Neurosurgery sometimes involves inserting an instrument through a burr hole or other entry portal into a subject's brain toward a target region of the brain. Because of the precision needed to reach the target, while avoiding nearby structures that are often critical to brain function, precise guidance devices and techniques are needed. In one such technique a multi-lumen instrument guide is included within a trajectory guide mounted to the skull. An instrument is inserted through a guide lumen of the instrument guide, which steers it toward the target.